Multipodal nanotubes and process for making same

ABSTRACT

Nanostructures, nanostructure arrays and a method of forming same are provided, wherein the nanostructures comprise ordered, self-organized, anodically formed single nanotubes, multipodal nanotubes or a combination thereof.

PRIORITY

This application claims priority of U.S. Provisional Patent ApplicationSer. No. 61/417,733, entitled “Multipodal Nanotubes and Process forMaking Same”, filed Nov. 29, 2010.

TECHNICAL FIELD

The present disclosure relates to the field of nanostructure arrays.More specifically, the present disclosure relates to nanostructureshaving increased hierarchical structures comprising single nanotubes,multipodal nanotubes and/or a combination thereof. The presentdisclosure also relates to an electrochemical anodization technique forcontrollably producing the foregoing nanostructure arrays.

BACKGROUND

The process of electrochemical anodization to form oriented nanotubesand nanostructure arrays is known. For example, anodization techniquesare commonly applied to form n-type semiconducting, near-verticallyoriented, self-organized titania (TiO₂) nanotube arrays. Such TiO₂nanotube arrays constitute a mechanically robust, often functionalizedarchitecture having a high surface area with vectorial electronpercolation pathways. Very similar anodization techniques have also beenused to form near-vertically oriented, self-organized nanotube andnanostructure arrays in other valve metal oxides such as, for examples,hafnium oxide, zirconium oxide and iron oxide.

The particular structure of nanotube and nanostructure arrays allows fortheir use in a variety of applications, including, without limitation,gas sensors, photocatalysts and scaffolds for excitonic solar cells.Other known applications, where the tubular structure and modifiablepore-size of the nanotubes array are the properties of interest, includedrug eluting coatings for medical implants, solid-phase microextraction(SPME) fibers and stem cell differentiation.

Despite significant progress in the field of nanotube and nanostructurearray production (i.e. the ability to tune the length, wall thicknessand diameter of nanotubes), a number of applications, including theaforementioned techniques, would benefit from the production ofnanostructures having a more complex topology than known titania orother nanotube arrays. For instance, applications and techniques thatmay rely upon volumetric filling or surface functionalization ofnanotubes might benefit from the production of multipodal nanotubes.

It is known that modifying the anodization parameters, such as, forexample, the voltage or temperature applied to the structures as theyform may produce more complex nanostructures. However, the impact ofmodifying anodization parameters remains relatively unknown. Attemptshave been made to increase the degree of complexity of nanostructuresthrough multi-step sonoelectrochemical anodization methods. Theresulting number and frequency of nanostructures may be increased bycausing the nanostructures to “branch”, i.e. to cause the nanostructureto divide, such that the parent nanostructure branches into a “Y-shaped”nanostructure having two identically-sized daughter nanostructures.Despite the foregoing process producing hierarchically branchednanostructures, however, the resulting structures are still limitedbecause the division of the parent nanostructure necessitates that thetwo or three daughter structures be identical in size. This is true evenwhere the nanostructure is “multi-branched”, such as having two, three,four, or even more branches.

There is a need for nanotubes and nanostructure arrays having a morecomplex topology, such as, for example, multipodal nanostructures. Thereis further a need for a controllable and reproducible method ofproducing nanotubes and nanostructure arrays having more complextopology.

SUMMARY

An electrochemical anodization method of producing nanostructure arrayshaving single nanotubes, multipodal nanotubes, or a combination thereof,is provided. A nanostructure array comprising a plurality of oriented,tapered nanostructures, wherein some or all of the nanostructures mayhave combined to be at least bipodal (i.e., having at least two “legs”),is further provided.

An electrochemical anodization method for producing a nanostructurearray having single nanotubes, multipodal nanotubes, or a combinationthereof, is provided, wherein the method comprises the steps of:

-   -   a. providing a substrate capable of undergoing anodization,    -   b. providing an electrolytic solution for receiving the        substrate,    -   c. providing means for restricting the mobility of ions in the        electrolytic solution, and    -   d. anodizing the substrate to produce single nanotubes,        multipodal nanotubes or a combination thereof.

In one embodiment, the means for restricting, reducing or slowing themobility of ions may comprise providing an electrolytic solution havinga viscosity sufficient to restrict the mobility of ions within thesolution. In another embodiment, the means for restricting or slowingthe mobility of ions may comprise providing a substrate havingpre-existing nanostructures. In a further embodiment, the means forrestricting the mobility of ions may comprise providing an electrolyticsolution having a viscosity to restricting the mobility of ions, asubstrate having pre-existing nanostructures, or a combination thereof.It is contemplated that other means for restricting or slowing themobility of ions may be provided and/or combined with the meansdescribed herein.

In one embodiment, the present method may comprise providing anelectrolytic solution comprising a mixture of:

a. a solvent,

b. a halide-bearing species, and

c. de-ionized water.

A nanostructure array comprising a plurality of oriented, taperednanostructures, wherein some or all of the nanostructures have combinedto form multipodal (e.g. at least bipodal) nanotubes, is furtherprovided.

In one embodiment, the present nanostructures may comprise a pore size(e.g. diameter) of at least 150 nm.

Broadly stated, in some embodiments, a method of producing ananostructure array is provided, comprising: providing a substratecapable of undergoing electrochemical anodization to form at least onenanostructure, providing an electrolytic solution for receiving thesubstrate, providing means for restricting the mobility of ions in theelectrolytic solution and anodizing the substrate to form singlenanotubes, multipodal nanotubes and/or a combination thereof.

Broadly stated, in some embodiments, a nanostructure array is provided,wherein the array may comprise a plurality of oriented, combinednanostructures.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is SEM images of multipodal titania nanotubes anodized in adiethylene glycol (DEG) solution with 0.25% HF and 2% water (a) at 120Vfor 44 h, (b) at 120V for 47 h, and (c) and (d) at 150V after 47 h.Arrows in FIG. 1( a) point to multipodal nanotubes not obscured by thetopology or tilt angle;

FIG. 2( a) is an SEM image of the cross-section of titania nanotubesformed by anodization at 120V in a DEG electrolytic solution with 0.25%HF and 1% water showing a clear taper from the base to the mouth; FIG.2( b) shows anodic current density as a function of anodization time for120V anodization with identical DEG electrolytic solution (0.25% HF and1% water);

FIG. 3 is an SEM image of the surface of a Ti foil anodized in aDEG-based electrolytic solution containing 0.25% HF and 1% H₂O for 43hours at 120V. Two distinct regions consisting of close-packed andwidely separated nanotubes are demarcated by the delineated border;

FIG. 4 is SEM images of titania nanotubes anodized at 120V in aDEG-based electrolytic solution with 0.25% HF and 1% water after (a) 40h, (b) 43 h, (c) 45 h, and (d) 47 h;

FIGS. 5( a) and (b) are SEM images of titania nanotubes anodized at 120Vin a DEG-based electrolytic solution with 0.25% HF and 1% water (a)after 45 h of anodization and 1 hour in the same bath without electricfield (four consecutive bipodal nanotubes can be seen), (b) after 45 hof anodization; FIG. 5( c) is a graph showing pore size of theindividual and combined nanotubes vs. anodization time; and FIGS. 5( d)and (e) are schematic images of the pore size increment in individual(d) and combined nanotubes (e);

FIG. 6 is an SEM image of the surface of a Ti foil anodized in aDEG-based electrolytic solution containing 0.25% HF and 1% H₂O for 45hours at 120V. Two distinct regions consisting of close-packed andwidely-separated nanotubes are seen (showing more advanced chemicaldissolution of the widely-separated nanotubes than that observed in FIG.3). Many of the surviving nanotubes in the chemically etched region aremultipodal (circled);

FIG. 7 is an SEM image of closely compact TiO₂ nanotubes after 22 hoursof anodization;

FIG. 8 shows a cross-sectional view of the grown individual nanotubes asof 22 hours of anodization in which their tapered conical shape can beseen. This image demonstrates that none of the nanotubes are combinedduring or following the first step of anodization;

FIG. 9 is an SEM image of nanotubes produced by two-step anodization,wherein the first step consisted of a 22 hr anodization followed by asubsequent 19.5 hr anodization (no nanotube combination is observed);

FIG. 10 shows nanotubes produced by two-step anodization, wherein thefirst step consisted of a 22 hr anodization followed by a subsequent23.5 hr anodization. Nanotube combination is observed to be in theinitial stages as the randomly oriented straight lines are locationswhere nanotube combination is occurring or has already occurred;

FIG. 11 shows a line of combining nanotubes;

FIG. 12 shows several lines of combining nanotubes;

FIG. 13 shows lateral increment of the pore size due to the nanotubecombination process;

FIG. 14( a) shows an SEM image (top view) of titania nanotubes subjectedto 22 hours of anodization in the first step followed by a 19.5 houranodization in the second step (the typical outer diameter of thenanotube is ˜150 nm); FIG. 14( b) shows an SEM image (top view) oftitania nanotubes subjected to 22 hours of anodization in the first stepfollowed by a 23.5 hour anodization in the second step (the typicalouter diameter of the nanotube is still ˜150 nm);

FIG. 15 shows an SEM (top view) of nanotubes combining followinganodization in formamide,

FIG. 16 shows an SEM (top view) of close-packed multipodal nanotubesfollowing anodization in formamide;

FIG. 17 shows an SEM (top view) of separated nanotubes followinganodization in formamide (enlarged view); and

FIG. 18 shows an SEM (top view) of separated nanotubes followinganodization in formamide.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Nanotubes and nanostructure arrays having increased hierarchicalstructure are provided. More specifically, nanotubes having more complextopology, nanostructure arrays having increased hierarchical structureand a controllable process of making same are provided.

According to one aspect of the invention, the present anodizationtechniques may used to controllably fabricate a nanostructure arrayhaving an oriented, complex hierarchical structure and topology.One-step and/or multi-step techniques described herein may be used toform nanotubes having increased hierarchical topology, such as, forexample, multipodal nanotubes, as well as nanostructure arrays havingmore complex structures, such as, for example, nanotube arrays havingsingle nanotubes, multipodal nanotubes, or a combination thereof.

Anodization Process

Preferred methods of the present anodization techniques will now bedescribed with reference to FIGS. 1-18. It is contemplated that anyanodization technique that is capable of slowing or inhibiting masstransport and/or reducing the mobility of ions, may be provided. Inother words, without limiting the present technology in anyway, it maybe desirable to provide any anodization technique that is capable ofrestricting or constraining the transport/movement of ions through theelectrolytic solution (i.e. more onerous ion travel). The foregoinghindrance of mass ion transport may further prevent possible dissipationof ion gradients within the electrolytic solution, thereby preservingthe scarcity of certain ions at different areas along the nanostructureduring formation thereof.

It should be understood that known anodization one-step or multi-steptechniques may be utilized by way of the present method and that askilled person in the art would know and understand how such techniquesmay be modified to result in the present nanotubes having more complexstructure and/or nanostructure having increased hierarchical topology.One can further appreciate that there may be many features thatdistinguish the instant technology from known nanotubes, nanostructurearrays and methods of producing same. Indeed, the present embodimentshave been included to communicate the features of the design, structureand associated method of the techniques and are by way of example only,and in no way intended to duly limit the disclosure thereof.

The present anodization techniques may comprise providing a substratecapable of undergoing anodization. In one embodiment, the presenttechniques may comprise the use of a titanium foil. It should beappreciated that the material undergoing anodization need not betitanium, but may be a variety of other substrates, such as, forexample, silicon wafers coated with vacuum deposited titanium or glasssubstrates coated with titanium, or valve metals, including but notlimited to zirconium, iron, tantalum, niobium or hafnium. Further, it iscontemplated that the substrate may or may not include existingnanostructures thereon.

For example, the present techniques may comprise the use of a 0.25 mmthick titanium foil (99.7%, Sigma Aldrich). The titanium may be anodizedutilizing known two-electrode anodization methods. The two-electrodeanodization may be used having titanium foils as both the anode and thecathode. Alternatively, the cathode may comprise any number of metals,including, for example, platinum and gold, as would be known to a personskilled in the art.

The titanium foils may be dimensioned such that the anode is larger thanthe cathode. For example, the anode and the cathode may be dimensionedsuch that the anode is 1.25 cm×3.8 cm and the cathode is 0.6 cm×3.8 cm.It would be known to a person skilled in the art that the anode andcathode foils might only have half of their length immersed in theelectrolytic solution.

Prior to anodization, the titanium foils may be cleaned ultrasonicallywith soap, de-ionized water and isopropyl alcohol and then dried withnitrogen gas. Anodization may be carried out at room temperature.

The present anodization techniques may comprise an electrolyticsolution, for receiving the substrate.

In one embodiment, the electrolytic solution may comprise a solventhaving a viscosity sufficient to slow or inhibit the movement of ionsthrough the solution. For example, the present anodization technique maycomprise a solvent having a viscosity greater than water (1 cP) and atleast greater than 3 cP. In a preferred embodiment, the electrolyticsolution may comprise diethylene glycol (DEG; Fisher Chemical), ethyleneglycol or any other suitable organic solvent of viscosity greater than10 cP.

The electrolytic solution may further comprise a halide-bearing species.In one embodiment, the halide-bearing species may be selected from thegroup consisting of a fluoride-bearing species or a chloride-bearingspecies. In a preferred embodiment, the halide-bearing species may be afluoride-bearing species. For example, the fluoride-bearing species maybe HF (48% solution, Sigma Aldrich). The HF concentration of theelectrolytic solution may be less than 0.5%. In a preferred embodiment,the HF concentration may be between 0.25% and 0.35%. In anotherembodiment, the fluoride-bearing species may be NH₄F.

The electrolytic solution may further comprise de-ionized water. In oneembodiment, the concentration of de-ionized water may be at least 2%.

It is contemplated that the electrolytic solution may comprise anysolvent capable of dissolving a halide-bearing species. In oneembodiment, the electrolytic solution may comprise, for example,formamide as the solvent. Having regard to FIGS. 15-18, the use ofdifferent solvents, such as formamide, may result in differentinter-tubular spacings between the nanostructures, and further theproduction of regular, non-multipodal nanotubes of smaller diameter inthe “background” of the combined, multipodal nanostructures, therebydemonstrating a nanostructure array having a further increasedhierarchical structure.

The present anodization technique may comprise the application ofvoltage sufficient to create an electric field. In one embodiment, thetechnique may comprise the application of voltage of at least 70V. In apreferred embodiment, the voltage applied to the anodization may bebetween 120V and 150V. It should be understood that a skilled personwould know and understand the appropriate voltage level to be appliedfor the particular anodization technique being used.

It is contemplated that a multi-step anodization process may also beused to create the present nanostructure array. For example, in oneembodiment, the anodization process may occur by way of a first step,wherein the voltage applied is at least 10 V, and at least onesubsequent step(s), wherein the voltage applied is at least 35V. Inanother embodiment, the at least one subsequent step(s) may compriseincreasing the voltage during the course of the anodization. Forexample, the voltage applied may begin at at least 35V and increase toat least 50V (at a predetermined rate such as, for example, 1V every 5minutes).

The anodization process may be applied to the electrolytic solution fora duration sufficient to create nanotubes having increased hierarchicalstructure, and, more particularly, combined nanotubes. In oneembodiment, the anodization process may be applied to the electrolyticsolution for a duration of at least 20 hours. In a preferred embodiment,the anodization process may be applied for a duration of at least 40hours.

Where a multi-step anodization process may be used, the anodizationprocess may occur by way of a first step, wherein the duration of theanodization is at least 45 hours, and at least one subsequent step(s),wherein the duration of the anodization is at least 3 hours.

Following anodization, the substrate foils containing the nanotubes maybe cleaned by rinsing the foils with isopropyl alcohol and drying themin air. Subsequently, the foils may be placed into 0.1 M HCl acid for anhour and then dried in the oven for one hour at 100° C.

Morphology of the nanotubes including their length, diameter, wallthickness and separation may be investigated using a scanning electronmicroscope (SEM, ZEISS) as well as a field-emission scanning electronmicroscope (FESEM, JEOL 6301F).

Formation of Multipodal Nanotubes EXAMPLE 1 One-Step ElectrochemicalAnodization, DEG Solvent, Anodization Voltages of 120 V or Greater,Anodization Durations>40 Hours and HF Concentrations Lower than 0.5%

The present example demonstrates the use of the present anodizationtechniques to controllably produce oriented, complex nanotubes havingincreased hierarchical structure and topology. More specifically, thepresent example demonstrates a process referred to as “nanotubecombination”, whereby the present anodization results in “parent”nanotubes leaning towards each other and combining or conjoining to formmultipodal “daughter” nanotubes having at least two “legs” (see FIGS.1-14). According to one aspect of the invention, nanotube combinationmay result from the conjoining of two or more parent nanotubes to formone multipodal daughter nanotube having a larger pore size than each ofthe individual parent nanotubes.

The present example shows that the parent nanotubes need not be the samediameter, size or fabricated at the same time in order for nanotubecombination to occur. For example, two parent nanotubes having differentoriginal pore size and diameter that are fabricated during two separateanodization processes may combine to form a multipodal daughternanotube. It should be understood that the multipodal daughter nanotubemay be combined to comprise at least two “legs”, and indeed, maycomprise more than two “legs”.

FIG. 1 depicts the present anodically-formed multipodal titania (TiO₂)nanotube arrays, wherein a number of the visible nanostructures arebipodal (FIGS. 1( a),(b)), or even tetrapodal where two bipodalnanotubes have combined (FIG. 1( c)). Although the nanotube in FIG. 1(d) appears bipodal at first glance, it is actually tetrapodal and onlyappears bipodal because the process of nanotube combination for theconstituent bipodal nanotubes is complete.

As a result of nanotube combination, the present anodization process maybe utilized to fabricate a nanotube array having larger pore sizes thanknown anodization processes. Anodization in DEG-based solvents, forexample. appears to exhibit certain unpredictable and unusual featuressuch as the formation of hollow nanotubes with very large pore sizes (upto ˜900 nm) and discretization of nanotubes by large inter-tubularspacing.

Having regard to FIG. 2, a plurality of the nanostructures formed withthe present method, whether or not multipodal structures, may exhibit atapered conical or pyramidal appearance, having a wider base andnarrower mouth portion. The present nanotube combination process maythus also have a decisive role in the simultaneous increment of bothpore size and inter-tubular spacing of nanotube arrays formed by thepresent techniques, thereby providing a means of monitoring andmodifying inter-tubular spacing. Such control over the nanostructure maypotentially provide an advantage over, and deviation from, theclose-packed architecture resulting from known anodization techniques.

It is hypothesized that the “leaning” or “angling” of the nanostructurestowards each other may be as a result of the forces produced during thepresent “time-dependent” anodization. Indeed, the current nanotubebehaviour during the present anodization process appears to result innanotube combination (see FIGS. 5 a-5 e).

By way of background, field-assisted oxide dissolution and cationmigration, field-assisted oxidation of Ti and chemical etching are thecompeting reactions responsible for the growth of TiO₂ nanotube arrays.

The field-assisted reactions occur on either side of the barrier layerat the base of the nanotubes and are responsible for driving the Ti/TiO₂interface deeper into the Ti foil, a process that increases the lengthof the nanotubes. Chemical etching shortens the length of the nanotubes.The relevant chemical equations are as follows:

Field assisted oxidation: Ti+2H₂O→TiO₂+4H⁺+4e⁻  (1)

Field assisted migration: Ti⁴⁺+6F⁻→[TiF₆]²⁻  (2)

Field assisted dissolution: TiO₂+6F⁻+4H⁺→[TiF₆]²⁻+2H₂O   (3)

Chemical dissolution: TiO₂+6HF→[TiF₆]²⁻+2H₂O+2H⁺  (4)

Having regard to FIG. 2( a), nanotubes formed in HF bearing DEG-basedelectrolytes at large anodization potentials appear to exhibit a taper,wherein the nanostructure comprises a wider base and a narrower mouth.This taper may occur as a consequence of the significant variation inthe conductivity of the electrolyte resulting over the course of theanodization process.

The low conductivity of the DEG-based electrolyte has been remarked uponby others and occurs due to a combination of three factors:

-   -   a) The high viscosity of DEG and the concomitant low ionic        mobilities    -   b) Low concentration of ionic charge carriers due to low        dissociation of the weak acid (HF) and    -   c) Large hydrodynamic radius of dissociated ions due to        solvation by water and DEG molecules.

As the anodization of Ti proceeds, the concentration of [TiF₆]²⁻ ionsincreases with time due to the chemical reactions represented byequations (2), (3) and (4). Due to a more delocalized distribution ofcharge in the complex, [TiF₆]²⁻ ions are also less solvated andtherefore more mobile. Consequently, the conductivity of the electrolyteincreases with anodization duration, which manifests itself in a higheranodization current density at the same potential, an effect clearlyseen in the anodization current transient plot of FIG. 2( b), during thefirst 20 hours of anodization. The increase in the conductivity of theelectrolyte may result in a large proportion of the applied anodizationpotential available for the anodization process since the potential dropacross the electrolyte (anodization current i x electrolyte resistanceR) reduces with time. Therefore, the base of the nanotubes, which formslater in the process, may experience a higher effective anodizationvoltage than the top (mouth) of the nanotubes, which are formedrelatively early in the process. Due to the well-known dependence of thediameter of the nanotubes on the anodization voltage, a tapered nanotubemorphology wider at the base than at the top is produced.

The field-assisted oxidation process generates ions according toequation (1) and results in local acidification at the pore bottom. Onthe other hand, F ion starvation occurs where F ions are consumed bydissolution reactions. Therefore, while F concentration is maximum atthe mouth of the tubes (nearly equal to concentration in the bulkelectrolyte) and drops to a minimum at the pore bottom, H⁺ ionconcentration is maximum at the pore bottom and decreases towards themouth of the tubes. Such a fluoride ion concentration gradient along thelength of the nanotubes has been confirmed by compositional analysisusing X-ray photoelectron spectroscopy (XPS). It is known that lessfluoride results in a thick oxide layer which suppresses the transportof titanium, oxygen and fluoride ions, and excess fluoride results in athin oxide layer which enhances the transport of titanium, oxygen andfluoride ions, thus inducing inward growth faster. Nanotube lengthincreases so long as the rate of movement of the Ti/TiO₂ interface isfaster than the rate of loss of TiO₂ nanotubes by chemical etching. Theanodization current is roughly proportional to the strength of thefield-assisted reactions and is thus indicative of the rate at which theTi/TiO₂ interface is moving into the Ti foil.

As shown in FIG. 2( b), the anodization current in the presentanodization technique increases for the first ˜20 hours of anodizationand then decreases nearly monotonically. The increase in anodizationcurrent may occur due to an increase in electrolyte conductivity overtime. Thus, the rate of movement of the interface peaks at ˜20 hoursinto the anodization process and appears to decline thereafter due to apaucity of fluoride ions at the pore bottom. At this point in theanodization process, field-assisted dissolution weakens relative tofield-assisted oxidation, resulting in an increase in the thickness ofthe barrier layer. The thicker barrier layer retards the solid stateionic transport of reactants through the barrier layer and causes adecrease in the anodization current density. If purely high field ionicconduction was involved, then the current would be expected tocontinuously decrease with time. If purely mass transport control wasinvolved, the anodization current would be expected to level off insteadof decreasing. In our scheme, it is believed that the anodizationreaction may be under mixed control of the high field solid state ionictransport and mass transport. Chemical etching, in contrast, isrelatively constant and becomes more dominant as the anodization currentdecreases.

A mechanism is proposed that explains the foregoing observations andaccounts for the unique formation of multipodal TiO₂ nanotubes inHF-DEG-water electrolytes in the foregoing example. A salient feature ofthe DEG-based bulk electrolyte is its high viscosity (η_(DEG)=32 cP at298 K) which prevents its penetration into the inter-tubular spaces ofclose-packed nanotube arrays. Further, a gradient in fluoride-bearingspecies exists along the length of the growing nanotube, with thehighest concentration corresponding to that of the bulk existing at themouth of the tube and decreasing toward the barrier layer. Thus, in thefirst 20 hours of the anodization process, when nanotubes are increasingin length, chemical etching, even though isotropic, only shortens theheight of the nanotubes by etching from the top. The solid statetransport of reactant ions through the barrier layer occurs through ahigh-field process exponentially dependent on the electric field acrossthe barrier layer and therefore sensitive to barrier layer thickness.When the anodization current begins to decrease after 20 hours, there isincreased competition for the lower current from all the nanotubes andminor variations in barrier layer thickness play a significant role inallocating current among nanotubes. As chemical etching becomes moredominant, nanotubes in regions where the barrier layer is slightlythicker grow into the Ti foil more slowly but experience the same rateof chemical etching, thus gradually becoming shorter than nanotubes inregions where the barrier layer is slightly thinner.

Due to the tapered structure of the nanotubes, a decrease in the heightof such nanotubes also increases inter-tubular spaces where the viscouselectrolyte can now penetrate-thus the same nanotubes experience moreaccelerated rates of dissolution due to chemical attack from the sidesin addition to etching from the top. Soon, these nanotubes arecompletely consumed. Also, since the Ti/TiO₂ interface in the regions ofthicker barrier layer moves into the metal more slowly, these regionsare gradually more elevated with respect to adjacent regions with athinner barrier layer (see FIG. 3, which shows two such regions adjacentto each other). The region enclosed by the delineated border in FIG. 3has relatively close-packed nanotubes as well as dark regions indicativeof depth and greater topographic contrast. The barrier layer is visiblein the region outside the delineated border, which is lighter on accountof being at a higher elevation. In this elevated region, severalnanotubes have been consumed by chemical etching resulting in a widerseparation. Several of the still-remaining nanotubes in this region haveexperienced severe sidewall etching (some of these are pointed out bythe arrows in FIG. 3).

FIG. 4 shows SEM images of the obtained titania nanotube arrays atdifferent anodization times. After 40 hours of anodization (FIG. 4 a),the nanotubes are still fairly close-packed but from this point onward,chemical etching becomes dominant. FIGS. 4( b), 4(c) and 4(d) show thatthe nanotube structures become successively less close-packed in thecourse of the next few hours resulting in a dramatic decrease in theareal density of nanotubes on the substrate. The absence of sidewallchemical etching in the regions of closely compacted nanotubes wherehighly viscous electrolyte cannot penetrate into the intertubular spacespreserves those nanotubes. However, as can be seen in FIG. 4( a),despite the closely packed structure, there are separated regionsthroughout the sample facilitating their sidewall etching due to theelectrolyte penetration which increases the bare area in those regionsat longer times as shown in FIGS. 4( b), 4(c) and 4(d).

Nanotubes of very large diameter (extending to optical and near-infraredwavelengths) may be obtained in DEG-electrolytes as seen in FIGS. 5( a)and 5(b). Two concurrent processes may be responsible. Although closelypacked nanotubes do not seem to undergo chemical etching of theirsidewalls, they do appear to experience etching from the top, whichshortens them because of the presence of the electrolyte at their mouth.Type I nanotubes of large pore-size are formed by the top etchingprocess, which increases their diameter due to their tapered conicalshape. Type II nanotubes of large pore-size form by the combination ofnanotubes having smaller pore size. As shown in the diagram of FIG. 5(c), the pore size of both Type I (individual nanotubes) and Type II(multipodal nanotubes) was increased for longer anodization timessubsequent to the formation of the self-organized nanotubular structureson the surface growing from just over than 300 nm after 40 h ofanodization to about 900 nm after 47 h for combined nanotubes. Thereason for the pore size increment of the Type I nanotubes is shownschematically in FIG. 5( d).

The nanotube combination process is schematically shown in FIG. 5( e) inwhich FIG. 5( e) (step I) represents the common surface area of the twoadjacent nanotubes that are “leaning” towards one another. As with, forexample, ethylene glycol (EG) and water, DEG is a highly structuredsolvent with a three-dimensional spatial network of hydrogen bonds. Itis known that H⁺ ions, OH⁻ (hydroxide) ions and glycoxide ions haveanomalously high conductance and mobility in these electrolytes due tothe proton jump mechanism. Halide ions, on the other hand, have muchlower conductance and mobility in EG and DEG. A consequence of thisasymmetry is that hydroxide ions and glycoxide ions consumed at theTi/TiO₂ interface during oxidation are replenished from the bulkelectrolyte more quickly than fluoride ions consumed in theelectrochemical dissolution of the barrier layer and this asymmetrybecomes more pronounced as nanotube length increases. Also, the bulkier[TiF₆]²⁻ ions produced at the pore bottom do not disperse quickly intothe bulk electrolyte due to their low mobility in the viscouselectrolyte and their coulombic attraction to the anode.

It should be mentioned that the chemical etching at the mouth of thenanotube occurs continuously and the high viscosity of the electrolytelimits the long distance movement of the dissolved material whichincreases the concentration of the dissolved material in theelectrolyte/nanotube interface region. As depicted in FIG. 5( e)(II),this highly saturated electrolyte etches both the side wall andinterface between the leaning nanotubes and the small amount ofsaturated electrolyte at the mouth becomes super-saturated andadditional dissolved nanotube material becomes deposited onto the innersurface of the nanotube (the electrolyte/nanotube interface). Hence, theside wall becomes thicker which results in the reduction of in chemicaletching rate relative to that of the nanotube inter-wall. The SEM imagesin FIGS. 5( a) and 5(b) clearly show this stage after 45 h ofanodization with the same electrolyte mentioned before. According toFIG. 5( e)(III), the difference in the chemical etch rate dissolves theinter-wall deeper into the nanotube which results in the nanotubecombination as seen in the SEM image in FIG. 1( d) for 47 h ofanodization.

As can be seen in FIGS. 5( a) and 5(b), if the leaning nanotubes possessa large interface extending from the top to the bottom, the resulting“combined nanotube” looks like a single large pore size nanotube. Where,for example, the interface does not extend all the way down, then thecombined nanotube may appear like a typical “branched” large diameternanotube possessing multiple “legs”. The number of such distinct “legs”may depend upon the number of nanotubes which combined to produce thelarger diameter nanotube.

FIG. 5( a) shows that during the first hour after voltage removal, thenanotube combination process was at its initial stage and the inter-wallbetween the nanotubes did not proceed deeply into the nanotubes.Nevertheless, at longer times, the nanotubes are completely etched andan irregular film is redeposited from the supersaturated electrolyte.

Bending and bunching of high-aspect ratio (>150) TiO₂ nanotubes grown influoride ion bearing glycerol-water electrolytes has been previouslyobserved due to surface tension effects during the drying process. Bysupercritical drying in CO₂, such bending has been minimized or eveneliminated. The SEM images in FIGS. 1( c), 1(d), 4(a), 4(b) and 6demonstrate that the present nanotubes may “bend” or lean towards oneanother before combining. The nanotubes formed in the present study havemuch lower aspect ratios of ˜5-25, therefore implying that the forcescausing the bending may be much larger.

Nanotube combination efficiently allocates scarce fluoride bearingspecies since two or more nanotubes can obtain fluoride-bearing speciesfrom the same puddle of bulk electrolyte after nanotube combination.Consequently, it is possible that multipodal nanotubes that obtainaccess to fluoride-bearing species in the bulk electrolyte by theprocess of pore combination may continue to grow (the Ti/TiO₂ interfacebelow them keeps moving deeper into the Ti metal). This is supported byFIG. 6, where multipodal nanotubes appear to survive longer than othernanotubes subsequent to the first 40 hours of anodization, whenfield-assisted processes weaken leaving chemical etching dominant. Thismay result in the weaker dissociation of HF, which results in fluorideion scarcity which may be the critical factor for nanotube combination.This is further supported by the observation that when ammonium fluoride(which has higher dissociation) and tetrabutyl ammonium fluoride (whichdissociates completely) are used instead of HF as the fluoride-bearingspecies in the diethylene glycol anodization electrolyte, F— ions arenot scarce and the formed nanotubes remain close-packed for very longanodization durations and the diameters are capped at ˜300 nm. Asmentioned previously, a gradient in fluoride ion concentrations existsalong the length of the nanotube with the maximum concentration in thebulk electrolyte close to the mouth of the nanotube. Viscouselectrolytes inhibit the rapid mass transport required for equalizationof the fluoride ion concentrations due to the low diffusion coefficientsof ions in them. Mass transport is also inhibited by the presence of apre-existing layer of vertically oriented nanotubes since such nanotubespresent narrow channels for flow of the relevant species and once againprevent dissipation of fluoride ion gradients generated due to theanodization process. Accordingly, it is contemplated that even in lessviscous electrolytes such as formamide (η=3.3 cP), nanotube combinationto form multipodal nanotubes can still occur in a controllable andreproducible fashion, provided a layer of nanotubes of diameter <150 nmwas already present to significantly slow down transport processes. Theresulting multipodal nanotubes are shown in FIGS. 16-18.

In summary, the formation of multipodal nanotubes in the foregoingexample is demonstrated using the following process parameters: DEGelectrolyte, anodization voltages of 120 V or greater, anodizationdurations >40 hours and HF concentrations lower than 0.5%. It is knownand understood that multipodal nanostructures may still be achieved as aresult of nanostructure combination with any number of these processparameters being modified.

One advantage of the multipodal nanostructure is that the differentialchemical functionalization of each individual “leg” or “pod” may allowfor multiplexed sensing and the loading of multiple drugs. Providingmore than one “leg” per nanostructure may result in a more robustattachment of the nanostructure onto a desired substrate, which couldalso render the nanostructures to be good load bearing elements formounting heavier structures. Further, the multipodal topology of thepresent nanostructures may also lend itself to the use of three-terminaldevices, electrical interconnect networks and nanoelectromechanicalsystems. The syntheses and applications of multipodal quantumdots—mainly tetrapodal nanocrystals of II-VI semiconductors such as ZnO,CdS and CdTe—are a focus of intense research activity. It follows thatthe present multipodal structure may provide certain advantages inapplications such as photocatalysis and photovoltaics due to the largersurface-to-volume ratio and more facile charge separation at thecore-leg interfaces. The hierarchical topology of multipodal nanotubes,consisting of multiple discrete nanotubes of smaller diameter combiningto form a single nanotube of larger diameter, could be applied for phaseseparation in a fluid comprising several ingredients and formicrofluidic and optofluidic applications.

EXAMPLE 2 Multi-Step Electrochemical Anodization, DEG Solvent,Anodization Voltages of 120V-150V or Greater, Anodization Durations>40Hours and HF Concentrations Lower than 0.5%

This experiment was done to explore the controllability of the nanotubecombination process by changing the time sequence of anodization and bymore closely observing the time period during which nanotube combinationis initiated. The main characteristic of this experiment was thatcontrary to the previous works the anodization was not performed in onecontinuous step, but rather performed in a sequence of steps such as,for example, two discrete steps.

The same electrolytic solution was used in this Example as in theprevious Example 1. For clarity, the present electrolytic solutioncomprised of 0.25% HF and 1% water in DEG. The voltage applied was 120V. The first step was identical to that used above up to the point thatthe combination process begins.

Previously, it was observed that the first 22 hours of anodization inthe electrolyte mentioned above resulted in closely compacted nanotubesas shown in FIG. 7. At this point in the process (when no sign ofnanotube combination was observed; see FIG. 8), the samples were takenout of the solution, rinsed in water, then in isopropanol, and thendried by a stream of nitrogen. The samples then underwent a cleaningprocess in 0.2 M HCl followed once again by rinsing and drying.

At this point, the samples which underwent the first anodization stepwere separated into two groups. One group of samples underwent asubsequent step of the anodization for 19.5 hours in a fresh electrolyteof the same recipe used before, whereas another group of samplesunderwent a subsequent step for 23.5 hours.

Where the anodization was performed continuously for >40 hours in thefirst step (and in Example 1), multipodal nanotubes were produced due tothe process of nanotube combination. However, where a first step (22hours) was combined with a second step (19.5 hours), no combinationoccurred (see FIG. 9).

Where, however, the anodization in the subsequent step was conducted for23.5 hours, the nanotube combination process appeared to be initiating.The alignment shown in FIG. 10 suggests that the nanotubes are likely inthe process of combining. Closer views of such combining lines aredepicted in FIGS. 11 and 12. Combination of adjacent nanotubes resultsin pore size increment in one dimension as shown in FIG. 13.

FIGS. 14( a) and 14(b) show the top views of nanotubes which weresubjected to anodization for 19.5 hours and 23.5 hours, respectively, inthe subsequent anodization step. The pore diameter of the nanotubesremains approximately the same in both cases (˜150 nm). It is likelythat, during the four hours of anodization separating the two samples,the process of chemical etching may have dissolved a significant amountof material near the mouth of the nanotubes. Since the nanotubes aretapered with a narrow diameter at the top and a wide diameter at thebase, the four additional hours of etching experienced by the 23.5 houranodized samples may have resulted in a larger tube-diameter. However,FIGS. 12 and 13 show that that this is not the case. Instead, despite ofthe four additional hours of etching, the 23.5 hour anodized sample hasnearly the same average nanotube diameter as the 19.5 hour anodizedsample. As such, it is contemplated that a multi-step sequentialanodization process may either arrested or severely slow the process ofchemical etching, thereby allowing greater control of the process ofchemical etching and the process of nanotube combination separately.This results in a substantial and previously unknown level of controlover the anodization process in general and over the nanotubecombination process in particular. It also shows a generalelectrochemical path towards the controllable combination of orientednanostructures on a substrate (where the said nanostructure might beformed by a variety of methods including but not limited to chemicalvapor deposition, solvothermal synthesis, vapor-liquid-solid growth,templated synthesis, electrochemical synthesis and photolithography).

EXAMPLE 3 Multi-Step Anodization, Titanium Substrate Having Pre-ExistingNanotubes, Formamide Solvent, Anodization Voltages of at Least 10V(First Step) and at Least 35V (Second Step), and Increasing to at Least50V at a Predetermined Rate, Anodization Durations of at Least 45 Hours(First Step) and 3 Hours (Second Step) and NH4F Concentrations Lowerthan 0.5%.

Having regard to FIGS. 15-18, the present Example was done to determinewhether nanotube combination could occur on a substrate havingpre-existing nanostructures, via a one-step or multi-step process. Thepresent Example supports the importance of restricting/constraining masstransport in the present process, particularly where a solvent havingrelatively low viscosity is used (e.g. formamide). It is contemplatedthat the slowing of mass transport occurred in the present Example as aresult of the pre-existing nanostructures on the array.

Step 1:

The first step of this Example 3 comprised anodizing a titaniumsubstrate at 10 V for 45 hours in a fresh formamide electrolyticsolution containing 0.3385% NH₄F and 5% H₂O. This first step resulted inthe formation of self-organized vertically oriented nanotubes with anaverage diameter of 50 nm and an average wall-thickness of 19 nm.

Step 2:

The second step of this Example 3 comprised taking the sample producedduring step 1 out of the electrolytic solution, and rinsing same withisopropanol and dried in a stream of nitrogen. The same sample was thenfurther anodized in a fresh formamide electrolytic solution containing0.3385% NH4F and 3% H2O in formamide. The anodization commenced at 35 Vand was increased at a predetermined rate of 1 V/5 minutes to 50V overan entire anodization duration of 3 hours and 20 minutes.

As is shown in FIGS. 15-18, the foregoing multi-step anodization processresulted in the formation of multipodal nanotubes with an averagediameter of ˜160 nm.

Although preferred embodiments have been shown and described, it will beappreciated by those skilled in the art that various changes andmodifications might be made without departing from the scope of theinvention. The terms and expressions used in the preceding specificationhave been used herein as terms of description and not of limitation, andthere is no intention in the use of such terms and expressions ofexcluding equivalents of the features shown and described or portionsthereof, it being recognized that the scope of the invention is definedand limited only by the claims that follow.

1. An electrochemical anodization method for producing a nanostructure array having single nanotubes, multipodal nanotubes, or a combination thereof, the method comprising: a. providing a substrate capable of undergoing anodization, b. providing an electrolytic solution for receiving the substrate, c. providing means for restricting the mobility of ions in the electrolytic solution, and d. anodizing the substrate to produce single nanotubes, multipodal nanotubes or a combination thereof.
 2. The method of claim 1, wherein the means for restricting mass transport comprises providing an electrolytic solution having a mixture of: e. a solvent having a viscosity sufficient to restrict the mobility of ions, f. a halide-bearing species, and g. de-ionized water.
 3. The method of claim 2, wherein the solvent viscosity is between 3 and 1000 cP.
 4. The method of claim 3, wherein the solvent is selected from a group consisting of diethylene glycol (DEG) and ethylene glycol.
 5. The method of claim 4, wherein the electrolyte is DEG.
 6. The method of claim 1, wherein the concentration of the halide-bearing species is less than 0.5%.
 7. The method of claim 6, wherein the concentration of the halide-bearing species is between 0.25% and 0.3%.
 8. The method of claim 6, wherein the halide-bearing species is a fluoride-bearing or a chloride-bearing species.
 9. The method of claim 8, wherein the halide-bearing species is HF.
 10. The method of claim 1, wherein the anodization occurs at a voltage of at least 70V.
 11. The method of claim 10, wherein the anodization occurs at a voltage of between 120V to 150V.
 12. The method of claim 1, wherein the anodization occurs for a duration of at least 40 hours.
 13. The method of claim 12, wherein the anodization occurs for a duration of 45 to 47 hours.
 14. The method of claim 1, wherein the substrate is titanium.
 15. The method of claim 1, wherein the substrate may contain pre-existing nanostructures.
 16. The method of claim 1, wherein the means for restricting the mobility of ions comprises providing a substrate having pre-existing nanostructures.
 17. The method of claim 16, wherein the electrolytic solution comprises a mixture of: h. a solvent, i. a halide-bearing species, and j. de-ionized water.
 18. The method of claim 17, wherein the solvent has a viscosity between 3 and 1000 cP.
 19. The method of claim 18, wherein the solvent is formamide.
 20. The method of claim 17, wherein the concentration of the halide-bearing species is less than 0.5%.
 21. The method of claim 20, wherein the halide-bearing species is a fluoride-bearing species or a chloride-bearing species.
 22. The method of claim 21, wherein the fluoride-bearing species is NH4F.
 23. The method of claim 17, wherein the anodization occurs at a voltage of at least 10V.
 24. The method of claim 17, wherein the anodization occurs for a duration of at least 40 hours.
 25. The method of claim 24, wherein the anodization occurs for a duration of 45 to 47 hours.
 26. The method of claim 17, wherein the method further comprises: k. Rinsing the nanostructure array, l. Performing at least one subsequent anodization.
 27. The method of claim 26, wherein the subsequent anodization comprises providing a second electrolytic solution comprising a mixture of: i. A solvent, ii. A halide-bearing species, and iii. De-ionized water.
 28. The method of claim 27, wherein the solvent viscosity is between 3-1000 cP.
 29. The method of claim 28, wherein the solvent is formamide.
 30. The method of claim 27, wherein the concentration of the halide-bearing species is less than 0.5%.
 31. The method of claim 30, wherein the halide-bearing species is a fluoride-bearing or chloride-bearing species.
 32. The method of claim 31, wherein the fluoride-bearing species is NH4F.
 33. The method of claim 26, wherein the subsequent anodization occurs at a voltage of at least 10V.
 34. The method of claim 33, wherein the subsequent anodization occurs at a voltage of 35V.
 35. The method of claim 26, wherein the voltage is increased to a voltage of at least 50V at a predetermined rate.
 36. The method of claim 26, wherein the subsequent anodization occurs for a duration of at least 3 hours.
 37. An electrochemical anodization method for producing a nanostructure array having complex hierarchical structure, comprising: a. providing a titanium substrate capable of undergoing anodization, b. providing an electrolytic solution for receiving the substrate comprising a mixture of: i. a solvent having a viscosity of at least 32 cP, ii. a fluoride-bearing species, wherein the concentration of the fluoride-bearing species is less than 0.5%, and de-ionized water, and c. anodizing the substrate at a voltage of between 120V-150 V for at least 40 hours.
 38. An electrochemical anodization method for producing a nanostructure array having a complex hierarchical structure, comprising: a. Providing a titanium substrate capable of undergoing anodization, wherein the substrate comprises pre-existing nanostructures, b. Providing an electrolytic solution for receiving the substrate comprising a mixture of: i. A solvent having a viscosity of between 3-1000 cP, ii. A fluoride-bearing species, wherein the concentration of the fluoride bearing species is less than 0.5%, and iii. De-ionized water, c. Anodizing the substrate in a first step at a voltage of at least 10V, d. Rinsing the nanostructure array, and e. Anodizing the substrate in at least one subsequent step at a voltage of at least 35V.
 39. A nanostructure array comprising: a plurality of oriented, tapered nanostructures, wherein some or all of the nanostructures may be at least bipodal.
 40. The nanostructure array of claim 39, wherein the plurality of nanostructures may comprise a pore size of at least 150 nm. 